The Sugary Fruit Paradox

A piece of fruit is more than the sum of its fiber and fructose.

Sugar is bad, but fruit is good. This seems like a contradiction, but every epidemiological study that has looked into it has found that higher levels of fruit consumption are associated with lower body weight and lower risk of obesity-related diseases, "with no evident upper threshold" – that's according to a Viewpoint piece in JAMA last month by David Ludwig of Harvard and Boston Children's Hospital.

How does this work? In general terms, the usual explanation is that fruit has lots of fiber that slows the rate at which the sugar is aborbed into the bloodstream compared to, say, juice or straight sugar. But I've always found this explanation a little puzzling, because it suggests that you could counteract the effects of any bad sugary food by consuming some fiber at the same time. Ludwig clarifies how this works in a New York Times interview:

“You can’t just take an 8-ounce glass of cola and add a serving of Metamucil and create a health food,” Dr. Ludwig said. “Even though the fructose-to-fiber ratio might be the same as an apple, the biological effects would be much different.”

Fiber provides “its greatest benefit when the cell walls that contain it remain intact,” he said. Sugars are effectively sequestered in the fruit’s cells, he explained, and it takes time for the digestive tract to break down those cells. The sugars therefore enter the bloodstream slowly, giving the liver more time to metabolize them.

So it's not just having sugar and fiber at the same time that matters – it's having the sugar locked away inside intact cells. That's one of the things you lose in transforming whole fruit into juice. And it's also another case where a whole food is fundamentally different than the sum of its constituent nutrients.